Photolithography is at present utilized for solder masks of some consumer printed wiring boards and almost all industrial printed wiring boards in view of high precision and high density. That is, it is a method using a photocurable resin composition which is exposed to active energy rays, subjected to a development treatment for image formation, and then further subjected to a finishing curing by heat and/or light irradiation. Besides, liquid solder masks of alkaline aqueous solution developing type using a diluted alkaline aqueous solution as a developing solution for development treatment predominate in consideration to environmental problems.
As such a solder mask of alkaline aqueous solution developing type, a solder mask composition composed of a photosensitive resin obtained by adding an acid anhydride to a reaction product of a novolac epoxy resin and an unsaturated monobasic acid, a photopolymerization initiator, a crosslinking agent and an epoxy resin, is disclosed in Patent Document 1.
However, the cured product of this solder mask composition is hard, and when it is applied to BGA (ball grid array) boards and flexible boards, whose usage is increasing in the fields of recent mobile devices, it causes trouble such as causing cracks on their surface and not following the bending of the boards.
As materials used for solder masks and cover lays applied to BGA boards and flexible boards, a composition using a compound obtained by reacting a reaction product of a polyfunctional bisphenol epoxy resin and a (meth)acrylic acid with a polybasic acid anhydride in view of flexibility, is described in Patent Document 2.
However, in the case where this material is used, although the crack resistance of the surface is improved, it has a problem in which the flexibility is still insufficient and the material cannot follow extreme bending.
Patent Document 3 and Patent Document 4 disclose, to further improve the flexibility, a photosensitive resin composition containing an unsaturated group-containing urethane resin obtained by reacting a hydroxyl group-containing unsaturated resin which is a reaction product of a diepoxide compound with 0.8 to 1.2 mol of an ethylenic unsaturated carboxylic acid having a number-average molecular weight of 72 to 1,000 and containing one average carboxyl group and one average ethylenic unsaturated group in one molecule to 1 mol of the epoxy group, a carboxyl group-containing diol compound, a diisocyanate compound, and optionally a polyol compound.
However, as for the resin design, raising the molecular weight causes increase in the acid value of the resin; raising the content of the ethylenic unsaturated group to enhance the sensitivity causes decrease in the acid value of the resin; thus, the molecular weight, sensitivity and developability are hardly balanced, and the composition has the problem with optimization as a solder mask composition.
Patent Document 5 and Patent Document 6 describe imide amide resins excellent in heat resistance.
However, these resins cannot be said to have the performance satisfying adhesiveness and flexibility.
Patent Document 1: Japanese Patent Laying Open (KOKAI) No. 61-243869
Patent Document 2: Japanese Patent No. 2868190
Patent Document 3: Japanese Patent Laying Open (KOKAI) No. 2001-33959
Patent Document 4: Japanese Patent Laying Open (KOKAI) No. 2001-33960
Patent Document 5: Japanese Patent Laying Open (KOKAI) No. 2000-344889
Patent Document 6: Japanese Patent Laying Open (KOKAI) No. 2000-344890
Printed wiring boards are demanded to have higher precision and higher density along with the reduced size and weight and the improved communication speed of mobile devices, so the demand for solder masks used therefor becomes increasingly high: concurrently having performances excellent in soldering heat resistance, resistance to electroless gold plating, adhesiveness to boards, chemical resistance and the like, while maintaining flexibility is demanded. However, there is at present no known composition fully satisfying these demands.
It is an object of the present invention to provide a photosensitive resin composition suitable for solder mask inks and cover layers, which is excellent in the photosensitivity to active energy rays so as to pattern fine images capable of meeting the high functionality of printed wiring boards, can form patterns by the development using a diluted alkaline aqueous solution, and provides a cured film, obtained by thermal curing in a post-cure process, having a sufficient flexibility, and excellent in the heat resistance, resistance to electroless gold plating, adhesiveness (to boards), chemical resistance and the like, and to provide its cured product.